1. Field of the Invention
The present invention relates to an optical fiber suitable for amplifying a long wavelength light signal, and more particularly, to a long wavelength optical amplifier whose gain and noise figure characteristics are improved by inserting an isolator between two erbium doped fibers.
2. Description of the Related Art
In an optical fiber communications system, transmission loss can be easily compensated for by using an erbium doped fiber amplifier (EDFA). The EDFA directly amplifies a light signal without converting the light signal into an electrical signal. U.S. Pat. No. 5,768,012 to Raymond Zanoni et al. entitled Apparatus And Method For The High-Power Pumping Of Fiber Optic Amplifiers and U.S. Pat. No. 5,673,129 to Victor Mizrahi entitled WDM Optical Communication Systems With Wavelength Stabilized Optical Selectors are examples of known optical amplifiers.
EDFAs are used to amplify weak light signals which have been transmitted over a long distance and carry a large amount of data, in order to compensate for the attenuation of the light signals when they are transmitted over a long length of optical fiber. In contrast with a transmission method in which light is converted into an electrical signal to be amplified and then converted back into light, since the EDFA can directly amplify light, it exhibits a good amplification efficiency without the need for optical-to-electrical conversion.
In general, EDFAs have a high gain in the range of 1525 to 1565 nm. If the length of an EDFA is sufficiently increased, a high gain can be obtained even in the range of 1570 to 1610 nm. Therefore, in order to increase the amplification band of an EDFA, two amplifiers having different amplifying bands may be employed.